class CreateViewport

The Viewport and View pair, together, represent the connection between two Flatland

instances. The Viewport is created in the parent, and the View is created in the child. The

parent has control over how the child's View is integrated into its own View.

Any illegal operations on ChildViewWatcher will cause both ChildViewWatcher channel and this

Flatland channel to be torn down.

`ViewportProperties` must have logical_size set. This is the initial size that will drive

the layout of the child. The logical_size is also used as the default Content size, but

subsequent changes to the logical_size will have no effect on the Content size.

`ViewportProperties` may have inset field not set. In that case, the default value of

(0, 0, 0, 0) is used.

The logical_size must have positive X and Y components.

Zero is not a valid ContentId. All other values are valid, assuming they are not already

in use for another piece of Content (see [`ReleaseViewport`] for more details).

Lifecycle note. The lifetime of the ChildViewWatcher channel is bound by the peer

ViewCreationToken. When the ViewCreationToken dies, this ChildViewWatcher channel is

destroyed.